Fabrication of thin film products such as microelectronic integrated circuits is enhanced by periodic measurements of key characteristics of the product during the fabrication process, enabling on-going process adjustments to enhance quality and yield. A prominent characteristic to be measured is thin film thickness at or around a specific location or a specific circuit element. Measurements of such characteristics as film thickness are best made by analyzing the wavelength spectrum of light reflected from the feature or location of interest on the workpiece or wafer. How to infer a measurement of a quantity such as film thickness from the wavelength spectrum is known. Many measurements may be desired during the processing of each individual wafer, so that the time required to perform each measurement reduces productivity. Such measurements must be made at predetermined precise locations (i.e., at user-selected devices in the integrated circuit, for example). Optical apparatus employed to capture a wavelength spectrum reflected from a specific or user-selected device or feature in the integrated circuit must be accurately focused on the exact location of that device or feature. The problem is that the movement or re-directing of the optical apparatus from one selected device to the next in the integrated circuit requires a significant amount of time. The movement must be precise and each selected feature must be located within an array of hundreds of thousands of features included in the integrated circuit.
One way this can be accomplished is to capture a digitized planar spatial image of a larger region of the integrated circuit that is most likely to contain the selected feature or device. This larger region may be a die or a portion of a die, and the precise location of the selected feature within the region is as yet unknown. Special pattern recognition algorithms are then employed to analyze the planar spatial image of the integrated circuit using the circuit design layout used to fabricate the integrated circuit. This analysis produces the exact location in the image of the selected circuit feature or device. This location may be specified as an exact X-Y location or a picture element (pixel) in the digital image. The optics is then used re-positioned to focus reflected light from the exact location discovered by the pattern recognition algorithm onto a diffraction grating. The spectrum of light emitted by the diffraction grating forms a wavelength-distributed intensity pattern along an axis of the grating, and this intensity pattern is focused onto a line sensor such as a charge coupled device (CCD) line imager. The output of the imager provides the reflection spectrum from the selected feature. Special wavelength analysis algorithms are employed to analyze this spectrum and infer from it a measured characteristic of the selected feature, such as thin film thickness for example. A limitation of this approach is that the mechanical re-positioning of the optics to each precisely determined location on the wafer is time consuming and must be performed for each successive measurement.
Another more sophisticated way in which thin film measurements at user-selected locations may be performed is to employ a spectral mapping and analysis of the entire region containing the user-selected feature. This latter approach eliminates the need to mechanically re-position the optics after capturing the image of the larger region. Specifically, the wavelength spectrum of each pixel of a large region most likely to contain the user-selected circuit feature is first obtained. Each row of pixels in the spatial image is passed through a line spectrometer grating whose output is focused on a CCD line sensor, producing columns of intensity values sorted by wavelength. This involves mapping each row of pixels in the spatial image into plural columns (one for each spatial image pixel) of spectral intensity values. Special algorithms analyze the spectra of all the pixels in the image of the large region and note contrasts in wavelength responses between different spatial regions. These contrasts point to boundaries between adjacent regions each containing common circuit features that differ from the common circuit features of the adjoining region. The locations of these boundaries may be correlated to the circuit design layout used to fabricate the integrated circuit. This correlation provides a precise mapping of locations in the image of the large region of the integrated circuit to features in the circuit design layout. From this mapping, the location of the user-selected feature or device is immediately deduced, identifying the exact pixel in the image of this feature. The wavelength spectrum of that pixel was previously obtained during the prior acquisition of the wavelength spectra of all pixels in the image of the large region. Therefore, the spectra of the identified pixel is simply fetched and provided for use by a special wavelength analysis algorithms to analyze this spectrum and infer from it a measured characteristic of the selected feature, such as thin film thickness for example. While this second approach eliminates the need for any mechanical repositioning of the optics or to focus the optics on any particular pixel, it is limited because the initial step of processing an array of wavelength spectra of all pixels in the image of the large region is computational intensive and represents a very large burden.
What is needed is a way of rapidly measuring plural user-selected circuit features on a wafer without having to re-position optics to each feature location and without imposing a large computational burden.